Bug 5860: Adding duplicate stocknumber fails silently

Bug 5860: Adding duplicate stocknumber fails silently

When adding a duplicate stocknumber the duplicate was not saved to the database.
This happened, because the field stocknumber in table items was declared to be unique.
Discussion on bug suggests to lift this restriction.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Tested. Corrected typo in updatedatabase script. Version 3.01.00.095 introduced the index with double s, although structure had only one s in index name.
Unaware of this bug, I entered the more general bug 6041 for similar problems with modifying items.
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>